Parkinson disease (PD) is the second most common neurodegenerative disorder. More than 10 million individuals worldwide are expected to have PD by the year 2030, and currently there are no known therapies that slow or halt the relentless progression of the disease. Dominant missense mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common known cause of PD, with up to 40 to 60 thousand carriers of the pathogenic G2019S-LRRK2 mutation in the United States. In individuals of Ashkenazi or Berber descent, frequencies of the G2019S-LRRK2 mutation can be as high as 20-30% in PD patients. In our last project period, we demonstrated that G2019S- LRRK2 expression promotes dopaminergic neurodegeneration caused by ?-synuclein over- expression and that LRRK2 kinase inhibitors blocked these effects. However, the mechanisms linking LRRK2 to ?-synuclein neurotoxicity are not clear. Further dissection of LRRK2 action in PD may expedite the discovery of new therapies that target LRRK2 and related pathways for neuroprotection. We found that LRRK2 is expressed in many neurons tougher with ?-synuclein in healthy brains, but in diseased brains that have ?-synuclein inclusions, LRRK2 is also expressed in myeloid cells that can direct the production of neurotoxic cytokines and reactive-oxygen species. In this competitive renewal request, we hypothesize that LRRK2 expression and kinase activity promotes myeloid cell recruitment to neurons that have ?-synuclein inclusions to exacerbate neurodegeneration and the spread of ?-synuclein inclusions. Using conditional transgenics that inducibly over-express G2019S-LRRK2 in myeloid cells, we will test whether G2019S-LRRK2 can upregulate cytokine responses and oxidative damage caused by neuronal ?-synuclein inclusions through promoting myeloid cell recruitment. Using conditional deletion technology, we will test whether LRRK2 knockout in myeloid cells will downregulate neurotoxic cytokines and oxidative damage in response to ?-synuclein inclusions through inhibiting myeloid cell recruitment. We will explore how LRRK2 protein can control myeloid cell motility and chemotaxis by acting as a novel scaffold for Arp2/3 to control actin polymerization. We predict that LRRK2 interacts with Arp2/3, in a kinase-dependent manner, to facilitate actin microspikes at the leading front of mobile myeloid cells to enhance motility. Completion of these studies should allow for further insight into how LRRK2 can cause PD, how LRRK2 expression in myeloid cells might contribute to neurodegeneration, and definition of a novel actin nucleation complex important for myeloid cell chemotaxis.